1. Field of the Invention
The invention relates to embedded inductor devices, and in particular to three dimensional inter-helix inductor devices with high quality factor.
2. Description of the Related Art
Embedded inductor devices have been applied in various circuits including resonators, filters, and matching networks. Among applications of wireless communication, digital computers, portable electronics, and information household appliances, product features with higher frequencies, broader bandwidths, and miniaturization have become main requirements by those associated with the high-tech industry and commercial markets.
For a system module, inductor devices are considered as a divide for radio frequency (RF) application and digital application. Applied in an RF module, conventional inductor devices are dependent from RF circuit matching and energy loss. The decisive parameters affecting inductor performance are self-resonance frequency (SRF) and quality factor. High self-resonance frequency can broaden operational band of the inductor device, while high quality factor can reduce signal transmission losses. Since the inductor devices are operated at high self-resonance frequency, characteristics of the inductor devices are changed and dominated by capacitance response. This can severely affect characteristics and performance of a circuit and a system module. Therefore, a need exists to reduce parasitic effect on the inductor or design of a novel inductor structure.
Typically, quality factor of inductor devices can be defined as shown in Eq. 1. More specifically, quality factor means the ratio of storage energy to dissipate energy during a periodic cycle.
                    Q        =                              2            ⁢            π            ×            The            ⁢                                                  ⁢            maximum            ⁢                                                  ⁢            stored            ⁢                                                  ⁢            energy                                The            ⁢                                                  ⁢            energy            ⁢                                                  ⁢            dissipated            ⁢                                                  ⁢            per            ⁢                                                  ⁢            cycle                                              Eq        .                                  ⁢        1            
The quality factor of an inductor device can be acquired by band width measurement, as expressed by Eq. 2.Q=F0/ΔF; F0: Operation frequency ΔF: 3 dB bandwidth  Eq. 2
Further, the quality factor of a inductor device is dependent from the equivalent series resistance (ESR) thereof. If the ESR is relatively small, the quality factor will increase for the same inductor mechanism. Moreover, distribution of electromagnetic field can also affect the quality factor of the inductor device. Surface roughness and process variations can also affect the quality factor of the inductor device.
When designing an embedded inductor device, therefore, considerations include desirable inductance of the embedded inductor device, grounding effect on the embedded inductor device, or electromagnetic field distribution. Conventional embedded inductor devices usually utilize large circuit layout area to achieve desirable inductance characteristics. On the other hand, when designing two-port inductor devices, circuit layout complexity become perplexed due to a far distance between input end and output end. The circuit layout area is also relatively increased. Moreover, since complexity of advanced communication system is continuing to increase, more inductor devices are needed to maintain circuit performance. Thus, improved embedded inductor devices are being demanded, but still elude those skilled in the art who are unable to meet demands and reduce circuit layout area and production costs.
U.S. Pat. No. 5,461,353, the entirety of which is hereby incorporated by reference, discloses a tunable embedded inductor device. Referring to FIG. 1, a tunable coil 10 is embedded in a multi-layered substrate structure. A transistor 18 is controlled by a control signal from a control line 15 to electrically short two adjacent conductive interconnections 14 and 16, thereby regulating inductance of the coil 10. Metal layers, functioning as shielding inductance, are disposed on the top and bottom of the multi-layered substrate structure, respectively. The advantageous feature of the tunable embedded inductor device is turning inductance with superb quality factor due to distribution of electromagnetic field confined within the spiral coil. Large circuit layout area, however, is needed to achieve coils with high inductance. Since the input end and the output end of the coil are separated by a very far distance, a very large circuit layout area is required for fabricating the two-port inductor device.
Further, U.S. Pat. No. 5,978,231, the entirety of which is hereby incorporated by reference, discloses an integrated coil inductor device. A magnetic material is pressed between two substrates, and a spiral inductor structure is formed on the magnetic material. Inductance of the spiral inductor structure is thus improved. FIG. 2A is a plan view of a conventional integrated coil inductor device, and FIG. 2B is a cross section of the integrated coil inductor device of FIG. 2A. Referring to FIGS. 2A and 2B, an integrated coil inductor includes a magnetic material layer s and interposed substrates. An embedded spiral inductor is disposed on the magnetic material layer s. The embedded spiral inductor is a coil structure consisting of conductive layers 29, 30 and conductive interconnection 28. More specifically, the coil structure includes conductive segments 29a, 29b and 30a, 30b disposed on both side of the magnetic material layer s, respectively. The conductive segments 29a, 29b and 30a, 30b are connected by conductive interconnections 28a-28e winding an embedded spiral inductor. Since the magnetic material is wound by the embedded spiral coil, relative large inductance can thus be acquired. Further, since stronger electromagnetic flux is distributed in the embedded spiral coil, improved quality factor can also be achieved. Conventional two-port integrated inductors, however, have a large distanced input end and output end, thereby increasing circuit layout area, which hinders integration with other active and passive devices.
U.S. Pat. No. 6,696,910, the entirety of which is hereby incorporated by reference, discloses a two-layered planar inductor structure. Referring to FIG. 3, a two-layered planar inductor device 50 includes a circuit board 54 and a ground plane 64 disposed on the circuit board 54. Screw holes 75, 76 are disposed at the peripheral region of the circuit board 54. An embedded spiral inductor 52 includes a winding 58 and conductive interconnection 62 disposed in the central region of the circuit board 54. A high relative permeability material is used as a core of the spiral inductor 52, and the inductor device can thus serve as a transformer. Other circuit elements 68, 70, 72 and 74, such as conductive lines and conductive interconnections 66 are further arranged on the circuit board 54.
The inductance of the conventional two-layered planar inductor devices is affected by core magnetic material. The inductance can be improved. The electromagnetic field concentrated within the spiral inductor can have excellent quality factor. The inductor device thus formed, however, still cannot reduce the circuit layout area even if the input and output ends are disposed closer together.